The present invention relates to a hydraulic shock absorber having the capability of controlling a the height of vehicle, and being attached to a suspension system of a vehicle, for example, an automobile.
Generally, in a vehicle, for example, an automobile, a hydraulic shock absorber is provided between a sprung mass and an unsprung mass in a suspension system to thereby damp vibration of the spring, improving the ride quality and the steering stability of the vehicle.
However, with respect to a vehicle having a relatively large loading capacity, such as a van and a station wagon, a load on the vehicle may change greatly depending on the number of passengers and the number of articles carried. Vehicle height is dependent on a load of the vehicle, so that the ride quality and the steering stability of the vehicle may deteriorate as a load changes. Hence, it has been desired to develop a suspension system for a vehicle which is capable of automatically maintaining a predetermined vehicle height, regardless of load.
Unexamined Japanese Patent Application Publication (Kokai) No. 60-261713 discloses a so-called self-pumping type hydraulic shock absorber which comprises an oil tank and a reservoir, each having a high-pressure gas sealed therein, a pump means for supplying a hydraulic fluid in the oil tank into a cylinder by the extension and contraction of a piston rod, and a relief means for relieving a hydraulic fluid in the pump means and the cylinder to the oil tank in accordance with a change in position of the piston rod. In this self-pumping type hydraulic shock absorber, the pump means is operated by utilizing vibration of a suspension system during running of the vehicle, appropriately applying pressure to the cylinder and maintaining a predetermined length of an extension of the piston rod, to thereby automatically maintain a predetermined vehicle height.
However, the self-pumping hydraulic shock absorber disclosed in Kokai No. 60-261713 involves a problem as mentioned below. When an empty vehicle provided with the above-mentioned self-pumping hydraulic shock absorber is running, in the self-pumping hydraulic shock absorber, the pressure of the hydraulic fluid in the cylinder is low, due to the small load on the vehicle. When this vehicle is loaded, that is, the load on the vehicle increases, the vehicle height lowers by an amount corresponding to the increase in load on the vehicle. When the vehicle runs after loading, the pump means is operated and a hydraulic fluid in the oil tank is supplied to the cylinder, so that the vehicle height gradually increases until it reaches a predetermined height. However, due to the time required to complete pumping, the vehicle height is still low at an initial stage of running of the vehicle, so that initial road clearance may be too small with resultant bottoming of the suspension system. Therefore, when a load on the vehicle is large or the vehicle is running along an uneven surface, bottoming of the suspension system may occur.